The invention relates to a probe for spectroscopic measurement of tissue.
Optical spectroscopy is a valuable tool for determining the chemical composition and concentration of specific analytes present in biological material. In vivo applications of optical spectroscopy to tissue include determining tissue pH, distinguishing cancerous from noncancerous tissue, and analyzing atherosclerotic plaque. Often, such a spectroscopic assay proceeds by measurement of a secondary parameter, e.g., an indicator compound for the condition of interest. The signal analysis may also proceed by relatively complex processing, such as correlation of spectral samples with an empirically-compiled database of tissue spectra. Much of the prior work in this area is accomplished by broad illumination of the sample and collection of reflected, transmitted or emitted light from a large area of the sample.
By way of example, detection of tissue pH by spectroscopic means has been found to be feasible, and to serve as a reliable indicator of tissue damage. Basically, under conditions of greatly reduced blood flow, hydrogen ions accumulate as anaerobic, metabolism commences, resulting in a tissue pH decline, indicating that the tissue is compromised. Irreversible ischemic damage may occur if tissue pH declines to 6.3 pH units or below. Continuous monitoring of tissue pH with suitably constructed electrodes has been known to provide valuable information to the surgeon to assess tissue viability during procedures such as heart surgery, and to monitor post-operative metabolism and blood flow. Spectroscopic monitoring as described for example in U.S. Pat. No. 5,813,403 offers a valuable alternative to such electrode-based monitoring. That patent, which is hereby incorporated herein by reference, describes a method and device for optical measurement of tissue pH. The method employs multivariate calibration techniques to relate reflected light to a reference pH measurement. For open surgery or endosurgical applications, near infrared spectroscopic measurement of tissue pH with such an instrument is useful in identifying regional areas of ischemia. This information can be used to evaluate myocardial protection and to monitor graft patency during or following heart surgery, and may be used to evaluate reperfusion injury during liver transplant procedures.
However a problem inherent in all in vivo tissue spectroscopy applications is that living tissue contains a great number of chemical compounds and physical bodies, and many uncontrolled processes of light absorption and scattering may interfere with the intended spectrographic assay.
It would therefore be desirable to provide an improved method and instrument for in vivo tissue spectroscopy.
This is achieved in accordance with one aspect of the present invention by a method of tissue spectroscopy, and an instrument for effecting such spectroscopy, wherein an illumination source and detector are arranged to provide a controlled light interaction path through tissue. The controlled path defines a limited interrogation volume, and may, for example, be effective to introduce an effective level of transmittance, for carrying out spectral or calorimetric measurements in the tissue with a minimal scattered component, or to provide effective scattering and transmittance components for determining both spectral and structural features of the interrogated tissue volume.
Both the illumination source and the detector are placed in contact with tissue, either directly or through the use of optical fibers, and an interrogation volume is defined by effective light paths that are determined, for a given set of tissue scattering and absorption properties, by the separation of the source and detector. This may be a banana-, sphere- or other shaped region, and has a limited volume that may be as small as about (0.015) cubic millimeters, or depending on the application, may amount to a volume of three, five or more cubic millimeters.
The size of the region may be initially set based upon structural dimensions of a tissue pathology or characteristic that is to be examined, so as to represent, for example, an expected tumor or tissue layer size dimension, maximumun penetration depth or the like, while the source-detector spacing is set to define a desired form of light interaction and interaction volume for the intended tissue measurement. A preferred system operates with illumination covering a broad band, or with plural discrete illumination sources at different wavelenths in a band, in the near infrared NIR region above about 400 nm.
In various embodiments, an instrument in accordance with the present invention has a probe body that positions the source and detector on or in tissue. An endoscopic or endovascular probe may have a body-insertable main fiber-carrying portion, with a side window or end window geometry, and the source-detector spacing may be adapted to spectroscopically determine tissue pH, cell distribution, or a range of colorimetric and other measurements.
Biological tissue is often heterogeneous and for many applications it is important to analyze light which interacts with a specific volume of sample, e.g., less than 3 mm3. Restriction of the light interaction to a small volume may be complicated by the highly scattering nature of tissue. In some applications it is important to isolate light that has interacted with a specific analyte and to minimize the component of detected light that has been scattered from cellular bodies; yet in other applications the scattered light contains biologically relevant information for the intended assay. A probe of the present invention is tailored to collect light from a specific volume of tissue. It can be fabricated to minimize or enhance the ability to collect scattered light, to collect light from a restricted volume such that the collected signal faithfully reflects an absorbance that is to be measured, or to otherwise enhance sensitivity to a feature of interest.
By collecting light from a very small region of tissue, the present device may enhance accuracy of the method described in U.S. Pat. No. 5,813,403. Specific source detector spacings, shape and positioning for the probe designs of the present invention may be selected based upon Monte Carlo modeling of light propagation through the tissue of interest and source/detector geometry is set accordingly. Prototype calculations for purely scattering media (Intralipid 10%), and for both absorbing compounds (e.g. hemoglobin) and the scattering media, are used to identify potentially suitable source-detector fiber separations, and different models provide source-detector separations that minimize scattering, and/or provide effective scattering information together with an effective transmission path length for spectrographic detection of specific constituents with appropriate localization of the tissue interaction volume for light entering and exiting the tissue. The probes may identify, or quantify components or constituents in the tissue such as hydrogen ions (pH), lactate, or an endogenous tissue component such as hemoglobin, myoglobin, lipids in plaque or the like.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein, including software User""s Manuals, are incorporated herein by reference in their entirety. In case of conflict, the present application, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.